The conventional mixture adjusting screw was so designed that it can be freely adjusted by the user even after the carburetor has been mounted in position in the engine, so that there was a risk that the screw would be improperly readjusted by the user to cause a change in the fuel efflux during idling and a corresponding change in the exhaust gas emission even beyond the level regulated by the law. Therefore, it is desirable to design the mixture adjusting screw such that it can not be randomly adjusted by the user once it has been properly adjusted by the maker.